For the past several decades, engineers have worked to develop apparatus and methods to effectively obtain information about downhole formations, especially during the process of drilling and following this process using wireline methods or pushed tool methods for use in horizontal wells. These methods may be collectively referred to as logging. During the drilling process and, with time afterward, drilling fluids begin to flush and intermingle with the natural fluids in the formation forming an invasion zone near the drilled borehole. This fluid exchange increases with time and the formation wall can degrade or become damaged with further drilling operations which can mask or alter information about the formation that is of interest. Logging-while-drilling (LWD) refers to a set of processes commonly used by the industry to obtain information about a formation during the drilling process. In some cases the acquired data from components located downhole on oil and gas drilling strings are transmitted to the ground's surface. Measurement-while-drilling (MWD) and LWD methods are also used in smart drilling systems to aid and/or direct the drilling operations and in some cases to maintain the drill in a specific zone of interest. The terms MWD and LWD are often used interchangeably in the industry and LWD will be used here to refer to both methods with the understanding that the LWD encompasses systems that collect formation, angular rotation rate and depth information and store this information for later retrieval and/or transmission of this information to the surface while drilling.
A common sensor used in logging systems is for the measurement of resistivity or the complement conductivity. The resistivity of the formation is quite often measured at different depths into the formation to determine the amount of fluid invasion and aid in the calculation of true formation resistivity. The formation resistivity is generally used with other sensors in an analysis to determine many other formation parameters. There are various types of resistivity sensors including direct current (DC), and alternating current (AC) focused resistivity which utilizes one or more electrodes devices, AC scanned resistivity which measures in a specific circumferential or angular pattern around the borehole and a fourth type called induction or propagation resistivity which also utilizes AC methods. Induction resistivity sensors generally use lower frequencies below 100 KHz while propagation sensors use higher frequencies. The terms induction sensor or induction tool will be used interchangeably here and will refer to both induction and propagation resistivity methods.
U.S. Pat. No. 6,677,756 to Fanini et al.; U.S. Pat. No. 6,359,438 to Bittar; U.S. Pat. No. 6,538,447 to Bittar; U.S. Pat. No. 6,218,842 to Bittar et al.; U.S. Pat. No. 6,163,155 to Bittar; U.S. Pat. No. 6,476,609 to Bittar; U.S. Pat. No. 6,577,129 to Thompson et al; U.S. Pat. No. 7,141,981 to Folberth et al; U.S. Pat. No. 5,045,795 to Gianzero, et al.; U.S. Pat. No. 5,606,260 to Giordano et al.; and U.S. Pat. No. 6,100,696 to Sinclair, each of which is herein incorporated by reference for all that it contains, disclose embodiments of downhole sensors that may be consistent with the present invention.
U.S. patent application Ser. No. 11/676,494, now issued U.S. Pat. No. 7,265,649 to Hall et al.; U.S. patent application Ser. No. 11/687,891, now issued U.S. Pat. No. 7,301.429 to Hall et al.; and U.S. patent application Ser. No. 12/041,754, now published U.S. Patent Publication No. 2008/0265892 to Snyder et al., each of which is herein incorporated by reference for all that it contains, disclose embodiments of induction resistivity tools.